The present invention relates to a developing device which makes an electrostatic latent image visible with developer, formed on an image carrier by means of electrophotographic recording or electrostatic recording. More particularly, the present invention relates to a toner concentration controller of a developing device in which dry type two-component developer is used, the toner concentration controller having a toner recycling means which recycles residual toner recovered from the surface of an image carrier by a cleaning means after transfer.
In an image recording apparatus such as an electrophotographic copier, a document is exposed to light so as to form an electrostatic latent image on the surface of an image carrier, then the electrostatic latent image is made visual by means of development, and the visual image is recorded on a recording sheet. Alternatively, in an image recording apparatus, an electric image signal is made visual, and the visual image is recorded on a recording sheet.
In this type image recording apparatus, two-component developer is used for the developing device so as to form a visual image, which is recorded on a recording sheet. In this two-component developer, toner is mixed with carrier at a predetermined mixing ratio. An amount of toner mixed with carrier is reduced as latent images are developed and recorded. When the toner amount is reduced, the density of recorded images is affected. Therefore, in order to obtain recorded images of a predetermined density, it is necessary to detect the toner concentration and to replenish toner when the toner concentration is reduced, that is, it is necessary to control the toner concentration in an appropriate range.
In this connection, in a developing device having a toner recycling means, recycled toner is supplied to an end portion of a developer conveyance screw so that new toner and recycled toner are stirred and mixed, and the mixed toner is conveyed to the development section.
In order to detect toner concentration, that is, in order to detect a mixing ratio of toner contained in developer, the following methods have been well known: a method in which the toner image density detected by a patch of reference density is optically detected; and an L (reactance) detection toner concentration sensor composed of a coil disposed to come into contact with developer so as to measure the permeability of developer.
According to the toner concentration detecting method in which the permeability is used for measurement, it is possible to detect the toner concentration at all times, and further the method is advantageous in that the image recording process is not changed for employing the method. According to the method, it is possible to maintain the toner concentration in an appropriate range when the detected toner concentration is compared with a toner reference value and a necessary amount of toner is replenished in accordance with the result of the comparison.
Concerning the L (reactance) detecting toner concentration sensor, for example, a toner concentration sensor referred to as a programmable toner sensor manufactured by TDK Co., Ltd. is known. The above toner concentration sensor is composed of an oscillation circuit including the aforementioned L coil, and a frequency-voltage conversion circuit. In this toner concentration sensor, the relation between toner concentration and output voltage is maintained to be negative characteristics in which: the higher the toner concentration, the lower the output voltage (shown in FIG. 10(A)). This L detection toner concentration sensor is provided with a control voltage input terminal. When a control voltage impressed upon this input terminal is changed, the output voltage of the concentration sensor can be adjusted.
FIG. 10(A) is a graph showing an example of the relation between control and output voltage in the L detection toner concentration sensor. FIG. 10(B) is a graph showing a change in the sensor output voltage with respect to the same toner concentration when control voltage is changed. In this case, the toner concentration is 5 weight percents.
[1] Initial reference value setting of the toner concentration sensor
(1) Developer of the standard toner concentration (for example, 5 weight percents) is loaded in a developing device. Then, the developing device is installed in a process unit. PA1 (2) Developer is stirred when the developing device is driven for 3 minutes. In this way, an amount of electrical charging of developer can be stabilized. PA1 (3) Control voltage (Vc) is changed, and a voltage V.sub.CO is selected at which the output voltage (V.sub.T) of the toner concentration sensor coincides with a control point (for example, 1.5 V shown in FIG. 10(A)).
[2] Toner concentration control
In a normal copy operation, the toner concentration sensor is controlled by the selected voltage V.sub.CO. The lower the toner concentration, the higher the sensor output, and toner is supplied until the toner concentration sensor output voltage returns to the control point V.sub.CO.
[3] Recycle toner
The initial reference value setting described above is conducted on the developer loaded into the developing device. Accordingly, in the case where the recycled toner has been returned from the cleaning unit in the above stage (2), the control voltage is selected under the condition that the toner concentration is increased by an amount of the recycled toner, and toner concentration control is conducted according to this toner concentration. Therefore, a mistaken toner concentration is stored in the developing device, so that an appropriate recording image density can not be provided.
The above problems are caused in the toner concentration control of the prior art.